In many cases, redundant circuits or a plurality of functions are also integrated in integrated circuits in order to allow them to be activated on demand. Redundant circuits of this type are activated if, as a result of incorrect processing (e.g., defects, particles), individual circuit parts are unable to function. The redundant circuits then perform the role of the defective circuits and the chip as a whole remains fully functional.
To activate the redundant circuit or function, the integrated circuit is electrically isolated from the defective region and connected to a redundant circuit (replacement circuit). This is achieved by fuses for disconnecting current paths and anti-fuses for connecting current paths.
An example of a disconnectable connection bridge (fuse) and a connectable line interrupter (anti-fuse) and of a process for producing and activating a fuse and an anti-fuse is disclosed by DE 196 04 776 A1.
These fuses have hitherto been integrated in one of the metallization levels of the integrated circuit. To break a fuse, a laser beam is directed onto the fuse and a short pulse of current is then responsible for melting through the fuse.
After processing of the chips has been completed, the chips are electrically tested for functionality before being mounted, for example in a housing. Non-functioning chips are repaired using the fuses as described above before being mounted.
If this described process is carried out directly by the manufacturer and if these chips (KGDs: known good dies) are mounted on substrates and over-molded, the fact that the fuses/anti-fuses are uncovered does not actually present problems, since the chip is sufficiently protected from environmental influences by the over-molding.
However, if chips are delivered to the customer in unpackaged form (bare chips or bare dies), and the customer then mounts the chips himself, considerable reliability problems are encountered. For example leakage currents, corrosion, erosion, etc. can result from environmental influences. There is then the considerable risk that fuses will be undesirably broken or anti-fuses will be undesirably connected, which can result in chip malfunctioning, even to the extent of the chip being rendered unusable.
To avoid these reliability problems, a silicon oxide or a silicon nitride or mixed layers can be deposited on the surface of the chip (wafer). But this approach would be very expensive and complex.